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Re: [TCML] "Means for increasing the intensity of electricaloscillations" The Tesla Superconductor of 1901
Hi Carl,
Carl Noggle wrote:
All true, except that I would disagree that the spark gap resistance is
a major factor. The current in the primary of a moderate sized TC (mine,
at 1600 watts, for example) is of the order of 100 amps. Going through a
spark channel that is a fraction of a mm in dia gives a very high
current density, not too different from lightning. A plasma at these
current densities has a resistivity that is lower than any metal. A
spark gap is a remarkably good switch for high currents.
A spark gap does indeed have a negative resistance characteristic, so
the higher the primary current, the hotter the plasma channel and the
lower the arc channel resistance. However, cathode and anode voltage
drops also contribute significantly to total gap voltage drop and
losses. The gap voltage drop is the sum of the resistive channel
("positive column") voltage drop plus cathode and anode voltage drops.
The cathode drop in particular is comparatively large. Both the anode
and cathode drops appear across the plasma sheaths that "connect" the
hot positive column to the comparatively cool gap electrodes.
A TC spark gap typically has a voltage drop of 150 - 250 volts depending
on electrode materials, gap current, and electrode/gap cooling. This is
not nearly as good as a metallic conductor or even saturated
semiconductor switches. This is why an SISG (with a series chain of
saturated IGBT switches) has a higher efficiency than an otherwise
identical spark-gap switched coil.
Voltage drops across multiple spark gaps add, so instantaneous power
losses across multiple gaps tend to scale with the number of gaps. A
single gap with high velocity air flow, such as Gary Lau's sucker/vortex
gap, provides higher efficiency than multi-gap systems since it
efficiently quenches while only using a single gap.
The main source of resistance in the pri circuit is the coil wire, due
to the very shallow skin depth. The bigger the better, stranded might
help a little, the best would be a lot of small insulated wires bundled
into Litz wire, but who wants to go to that much work?
There are calculators of resistance of wire at different frequencies on
the web. Check this URL--
http://ve3efc.ca/wire_ohms.html
The R of my primary (00 wire) from this table is about 0.07 ohms, and
since the impedance of the coil is about 100 ohms, wire resistance is
not a big factor. That means that without the secondary, the primary
circuit ought to have a very high Q. I think I'll check that on my coil.
By the way, let's forget Litz wire. Looks like I overspent on wire. But
it sure is a pretty coil.
If you actually measure your primary's Q (without any secondary
present), you'll discover some interesting behavior. The primary LC
circuit does not decay exponentially, but instead decays almost linearly
due to the combination of the gap's negative resistance characteristic
and the relatively fixed gap voltage drop. For an interesting discussion
about gap losses and measured efficiency of a single gap, search for the
"Vortex Gap Loss Measurements" thread in the Pupman archives:
http://www.pupman.com/listarchives/2000/September/msg00038.html
An interesting thing to be gleaned from the table is that the RF
resistance of any reasonable wire used for the secondary will have
hardly any increase of R at TC frequencies, so the ohmmeter resistance
of the secondary is the RF resistance. The secondary coil Q should be
high too. It seems that losses in the TC are small, and almost all the
energy should be going into the coronas. If you look though them in the
daytime, you can see quite a bit of heat distortion, even though it's
hard to see the sparks themselves.
If the radius of the wire is roughly the same as the skin depth, then
the HF resistance is approximately the same as the DC resistance.
However, TC secondaries are usually close-wound to maximize inductance,
and the RF resistance is usually significantly higher due to "proximity
effect". Similar to skin effect, this is due to magnetic fields from
adjacent windings further constraining current flow through the wire.
Proximity effect can cause boost the RF resistance by a factor of 2-4X
the DC resistance (or more). The combination of skin and proximity
effects make it challenging to wind TC secondaries with Q's much greater
than about 250 at resonant frequency. Again, the Pupman archives have
some good information - search "Proximity Effect". Also, chapter 2,
sections 18 and 19 in Terman's "Radio Engineer's Handbook" have a good
discussion and analysis of total copper losses (including proximity
effect) for single-layer air coils.
---Carl
Bert
--
Bert Hickman
Stoneridge Engineering
http://www.capturedlightning.com
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